Photodynamic therapy (PDT) , which is the interaction of light with hematoporphyrin derivative (HPD or Photofrin II) in tissue, is in clinical use in several areas of application. Currently, PDT is being used clinically for bronchial obstructions, carcinoma in situ of the bladder, and intraperitoneal and pleural tumors. The treatment light is provided by two high-powered argon ion/dye lasers, each capable of operating at 514nm or 630nm, and at two delivery levels. Since the lasers have RS-232 interfaces for control and monitoring, a computer makes an ideal device for remote control from the operating room. The computer can also provide timing for each segment, a permanent log of level versus time, and integrated energy delivered. It also allows the individual operating the light monitoring system to control the illumination.